In recent years, electronic appliances having a photosensor section utilizing a TFD, e.g., display devices and image sensors, are being developed.
For example, Patent Document 1 proposes forming semiconductor layers of a TFD and a thin film transistor (Thin Film Transistor: TFT), which are of different crystal states, from the same amorphous semiconductor film. Such a TFD and TFT are applicable to a display device having an optical touch-screen panel, for example.
In such a display device, TFTs are provided for respective pixels in a display region, and used as pixel switching elements (pixel TFT). Moreover, they are also provided in a frame region that is located around the display region, and used as driving circuit TFTs composing a driving circuit. On the other hand, TFDs are used as photodetection devices. For example, by providing photosensor sections including TFDs in the display region, it becomes possible to detect an object, such as a finger touching the screen, by utilizing external light.
Patent Document 2 proposes, in a display device having an optical touch-screen panel, performing sensing by using an invisible light beam which is emitted from a backlight, where sensors for detecting the invisible light beam are used. In the display device disclosed in Patent Document 2, an invisible light beam which is emitted from a backlight and is reflected from an object such as a finger touching the screen is detected by sensors which are provided in the respective pixels of the display device.
FIG. 11 is a circuit diagram showing the construction of a photosensor section which is disclosed in Patent Document 1. The photosensor section includes a thin film diode for use as a photosensor (hereinafter, a “photosensor TFD”) 701, a condenser 702 for signal storage, and a thin film transistor (hereinafter, a “buffer TFT”) 703 for allowing a signal stored in the condenser 702 to be taken out with amplification. After an RST signal comes in and an RST potential is written to a node 704, if the potential of the node 704 lowers due to leakage caused by light, the gate potential of the buffer TFT 703 will fluctuate, thereby causing the TFT gate to open or close. As a result of this, a signal VDD can be taken out.